The Structure of the Light Curves of the RR Lyrae Variables in the Oosterhoff Type I Cluster NGC 6171

CCD observations have been used to derive precise light curves and Fourier parameters for sixteen RR Lyrae variables in the Oosterhoff type I globular cluster NGC 6171. It is found that, for the RRab stars, the Fourier phase differences phi_ {21}, phi_ {31} and phi_ {41} all correlate with period, but the amplitude ratios R₂₁, R₃₁ and R₄₁ do not. For the RRc stars, R₂₁, phi_ {31} and phi_ {41} correlate with period, but phi_ {21}, R₃₁ and R₄₁ do not. It is also found that a plot of R₂₁ against phi_ {21} is a useful tool for distinguishing between RRc and RRab stars, regardless of color or chemical composition. Simon's method that relates physical properties of RRc stars to the pulsation period and Fourier phase parameter phi_ {31} has been used to determine the mass, luminosity, temperature and a helium parameter for the RRc stars. A comparison with the parameters determined from CCD observations of RRc stars in six other galactic globular clusters indicates that mean masses and mean luminosities increase and mean temperatures and helium abundances decrease with decreasing cluster metal abundance. The formulae of Jurcsik & Kovacs for relating Fourier parameters of RRab stars to [Fe/H] and luminosity have been tested and the results are reasonable.